1. Field of the Invention
The present invention relates to an electric power converter such as an inverter apparatus using a semiconductor element or a semiconductor module.
2. Description of the Related Art
In an electric power converter including a plurality of semiconductor elements or semiconductor modules and a cooler for cooling the semiconductors, the plurality of semiconductor elements or semiconductor modules are arranged on the cooler, and hence a cooling medium for cooling increases in temperature while flowing from an upstream side to a downstream side. Therefore, due to the increase in temperature of the cooling medium, semiconductor elements or semiconductor modules on the downstream side have stricter temperature constraints than those of semiconductor elements or semiconductor modules on the upstream side and are limited in power that can be applied thereto, which has posed problems in increasing output and reducing the size and cost of the electric power converter.
Meanwhile, there has been proposed an electric power converter in which two inverters and one converter are integrated (see, for example, Japanese Patent Application Laid-open No. 2009-296708). To be specific, a configuration is adopted in which a meandering groove is formed in a water jacket to constitute a cooling medium passage, and three power modules constituting a first inverter, three power modules constituting a second inverter, and a power module constituting a converter are arranged in the stated order in the cooling medium passage so that the semiconductor elements included in the power modules are cooled.
However, in the configuration of Japanese Patent Application Laid-open No. 2009-296708, the cooling medium is increased in the temperature due to heat generated by the first inverter, and the second inverter needs to be cooled by the cooling medium having the increased temperature. Therefore, the second inverter needs to be reduced in output as compared to the first inverter to reduce heat generated by the semiconductor elements, needs to be increased in a heat dissipation property to decrease the increase in temperature of the semiconductor elements, or needs to use semiconductor elements having a higher allowable temperature.
Note that, when the output is reduced, providing high power becomes difficult. When the heat dissipation property is increased, reductions in size (heat dissipation property is proportional to a heat dissipation area) and in cost (highly thermal conductive member needs to be used) become difficult. When the semiconductor elements having a higher allowable temperature are used, difficulty in reducing the cost (high heat-resistant member needs to be used) and other such problems arise in the converter.
In addition, due to the heat generated by the first inverter and the second inverter, the temperature of the cooling medium used to cool the converter is further increased than that for the second inverter, and hence bigger problems than those of the second inverter arise.
As a countermeasure against the increase in temperature of the cooling medium, there has been proposed a configuration in which semiconductor elements (insulated gate bipolar transistors: IGBTs), which generate a large amount of heat, and semiconductor elements (diodes), which generate a small amount of heat, are arranged on the upstream side and the downstream side of the cooling medium passage, respectively, to thereby reduce adverse effects of the increased cooling medium temperature (see, for example, Japanese Patent Application Laid-open No. 2007-12722).
However, the related technologies have the following problems.
In the configuration of Japanese Patent Application Laid-open No. 2007-12722, the IGBTs and the diodes are arranged in series with respect to the flow of the cooling medium, but the individual semiconductor groups (for example, the minimum units each constituted of a pair of an IGBT and a diode) constituting the electric power converter are all arranged in parallel with respect to the cooling medium passage.
In the electric power converter including one inverter as in Japanese Patent Application Laid-open No. 2007-12722, six cooling medium passages are formed in parallel, and accordingly a flow rate of the cooling medium that can be used by each semiconductor group is ⅙ as compared to the case where the series passage is adopted as in Japanese Patent Application Laid-open No. 2009-296708. When the flow rate of the cooling medium becomes lower, a heat dissipation performance of the cooler is reduced and further the increase in temperature of the cooling medium in each semiconductor group becomes larger.
Therefore, assuming a case where the configuration of Japanese Patent Application Laid-open No. 2007-12722 is applied to the high-capacity electric power converter in which two inverters and one converter are integrated as in Japanese Patent Application Laid-open No. 2009-296708, thirteen cooling medium passages are formed in parallel and the flow rate of the cooling medium that can be used by each semiconductor group is 1/13, with the result that the heat dissipation performance of the cooler is significantly reduced and the problems of Japanese Patent Application Laid-open No. 2007-12722 become more pronounced.
Moreover, the increase in temperature from an inlet to an outlet of the passage of the cooling medium is determined by a relationship between total generated heat from the inlet to the outlet and the flow rate. In the high-capacity electric power converter as in Japanese Patent Application Laid-open No. 2009-296708, the total generated heat from the inlet to the outlet is increased, but when the parallel passage configuration as in Japanese Patent Application Laid-open No. 2007-12722 is adapted thereto, the flow rate of the cooling medium of each semiconductor group is further reduced significantly.
Therefore, the increase in temperature of the cooling medium that occurs in one semiconductor group becomes very large to cause a large temperature difference in one semiconductor element, which leads to a deteriorated current distribution inside the semiconductor element, and further to new problems such as adverse effects on the electrical characteristics of the semiconductor element, such as a damage due to local heating of the semiconductor element caused by the concentrated current and a reduction in short circuit capacity.
Moreover, in Japanese Patent Application Laid-open No. 2007-12722, in order to arrange all the semiconductor elements at the uppermost stream of the cooling medium passages, it is proposed to arrange the semiconductor elements in parallel so that the semiconductor elements are not arranged in series in the individual passages, and to configure the passage so that the passage is bifurcated on the upstream side and the divided passages are merged after respectively having cooled one semiconductor element to flow to the downstream side.
However, in this configuration, the temperature difference between the upstream side and the downstream side of the cooling medium occurs while the cooling medium passes through only the respective ones of the semiconductor elements, and hence the temperature differences within the respective semiconductor elements become clearly larger than the above-mentioned example. In addition, it is necessary to arrange the semiconductor elements in line and to configure the passage so that the passage is bifurcated on the upstream side and the cooling medium is distributed to the respective semiconductor elements, and hence there have been cases where reduction in size is difficult and the flexibility in configuring the arrangement of components is low.
Moreover, in Japanese Patent Application Laid-open No. 2007-12722, as countermeasures against deterioration of thermal feasibility due to the increase in temperature of the cooling medium, efforts have been made to reduce the adverse effects of thermal interference between the semiconductors by arranging semiconductors on both surfaces of the cooler, arranging a diode, which generates a small amount of heat, immediately below an IGBT, which generates a large amount of heat, and other such efforts, to thereby improve the heat dissipation property. Thus, the efforts to reduce the adverse effects of thermal interference between the semiconductors and improve the heat dissipation property have been made, but a solution for the larger increase in temperature of the cooling medium in each semiconductor group due to the distribution of the flow rate of the cooling medium has not been proposed.
Moreover, in order to arrange the IGBTs to be connected to a control board for driving the semiconductor elements on upper and lower sides of the cooler, the connection configuration with the control board arranged on one side of the semiconductor module becomes complicated. In addition, the cooling medium passages are included in the semiconductor module, and hence as a countermeasure against a leakage of the cooling medium, the cooler, which includes the passages, and the semiconductor module need to be integrated, which results in difficulties in increasing the size of the semiconductor module and the capacity.